I have a 99 f-350 when I turn the key to the on position to heat glow plugs the #3 relay under the hood clicks and in turnt the service light and fuel pump turn on and off also a sensor on top of motor is making a hissing sound, fuel pump is a year old cam-posi-sensor is new, also the glow plugs aren't getting hot. Have replaced relay and would replace sensor if I knew what sensor it was.

So what exactly is the question? Is the truck not starting? I've had 7.3L fords and had the glow plugs go bad, and the truck still started no problem until cold weather set in, we need a little more info on exactly what your problem is.